A thermistor is a semiconductor, the resistive properties of which vary with the temperature. In order to enable the resistive properties of the thermistor to be utilized, it is provided with contacts that can be connected to an electric circuit. The resistance and temperature sensitivity of the thermistor are determined by the composition of the material of the semiconductor, the physical dimensions of the active substance of the thermistor, and the temperature.
The fact that the resistance depends on the physical dimensions of the material of the thermistor makes it possible to regulate the ohmic value of the thermistor by removing or trimming off some of the material. The resistance of the thermistor is also determined by the area of the contact surfaces on the thermistor material, which means that the ohmic value of the thermistor can be adjusted by removing or trimming off some of the contact surface on the material of the thermistor.
Different types of thermistor are known. In GB-A-1470630 a thermistor produced by a thick-film process is described. A first layer of contact material is applied to a substrate plate by screen printing, forming a number of pairs of electrodes. After firing, a second layer of thermistor material is printed on the first, to form a thermistor plate over the pair of electrodes. After refiring, the thermistor is trimmed by having part of the material removed with the aid of a laser. The substrate plate is divided into discrete thermistor elements and encapsulated in a protective layer of suitable material.
GB-A-1287930 describes a thermistor consisting of a first layer of contact material, a second layer of thermistor material fully encapsulating the first layer, and two electrode surfaces arranged parallel on the thermistor layer.
GB-A-1226789 shows a similar thermistor arranged on a substrate plate, which consists of a thermistor plate between a lower and an upper electrode surface. The electrode surfaces are extended in opposite directions on the substrate plate, in order to form contact surfaces for connection to an electric circuit.
None of the thermistors previously known is designed to be simply and very flexibly adaptable to different spheres of use while maintaining the possibility of high precision with the aid of exact trimming. This is essential to the production and trimming of the thermistors at a low enough cost for them to be usable as disposable products, such as disposable thermometers.